1. Field of the Disclosure
The present invention relates generally to power converter controllers, and more specifically, the invention relates to power converter controllers that are used for primary side regulation.
2. Background
Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. utilize power to operate. Because power is generally delivered through a wall socket as high voltage alternating current (ac), a device, typically referred to as a power converter can be utilized to transform the high voltage alternating current (ac) input to a well regulated direct current (dc) output through an energy transfer element. Switched mode power converters are commonly used to improve efficiency, size, and reduce component count in many of today's electronics. A switch mode power converter may use a power switch that switches between a closed position (ON state) and an open position (OFF state) to transfer energy from an input to an output of the power converter. In operation, a power converter may use a controller to provide a regulated output to an electrical device (generally referred to as a load) by sensing and controlling the output voltage and/or current of the power converter in a closed loop. A switching cycle may be defined by the switching frequency of the power switch. In one example, the duration of a switching cycle includes the power switch transitioning to an ON state followed by a time the switch transitions to an OFF state. The controller may be coupled to receive feedback information about the output of the power converter in order to regulate the output quantity delivered to the load. The controller regulates the output quantity delivered to the load by controlling the power switch to turn on and off in response to the feedback information to transfer energy pulses to the power converter output from a source of input power such as a power line.
For certain applications, a power converter may be required to provide galvanic isolation. Specifically, galvanic isolation prevents dc current from flowing between the input side and the output side of the power converter, and is usually required to meet safety regulations. One particular type of power converter that uses galvanic isolation is a flyback power converter.
One type of control method for galvanic isolated power converters is primary side control. Specifically, primary side control is when the controller uses a sensing element that is electrically isolated from the secondary side of the power converter. One type of primary side regulation may use an additional winding (e.g., a bias winding) that is electrically coupled to the input side and receives information from the output side through magnetic coupling. In one example, a bias winding may be used to sense an output voltage of a flyback power converter.
A power converter may be suitable to operate in two modes of operation. In a first mode of operation, known as discontinuous conduction mode, all the energy stored in the energy transfer element is transferred to the output during the OFF state of the power switch. In this mode of operation, there is a limited amount of time after the power switch is closed (in an OFF state) that bias winding voltage may represent the output voltage of the power converter. In a second mode of operation, known as continuous conduction mode, only a portion of the energy stored in the energy transfer element is transferred to the output during the OFF state of the power switch. In this mode of operation, the bias winding voltage may represent the output voltage for substantially the entire time the power switch is closed (in an OFF state). Even though the bias winding voltage may be representative of the output voltage for substantially the entire off time (time when power switch is in an OFF state) during continuous conduction mode, the duration of time that the bias winding voltage is representative of the output voltage is still shorter than when the controller is operating in a discontinuous conduction mode. This may occur, because the duration of the off time of the power switch in discontinuous conduction mode is long enough for all energy to be transferred to the output of the power converter; whereas the off time of the power switch in continuous conduction mode is truncated (to start the next switching cycle) before all energy is delivered to the output of the power converter. Therefore, when the controller is operating in continuous conduction mode, there is less time to acquire a sample of the bias winding voltage that is representative of an output voltage.
Furthermore, there may be switching noise coupled to the bias winding during the time the bias winding voltage is representative of the output voltage that even further reduces the window of time for the controller to capture a relatively ‘clean’ (without noise) signal from the bias winding that is representative of the output voltage of the power converter. Even still, under certain loading conditions (e.g high load demand), power converters may be required to increase power delivery by increase the switching frequency which further shortens the duration of time power switch is in an OFF state for each switching cycle. As a result, the sampling window for acquiring a clean signal is further reduced.